Method for producing an encapsulation for a SAW component operating with surface acoustic waves

ABSTRACT

An encapsulation for SAW components and a method for producing the encapsulation use a cap to seal component structures on a substrate. The cap is in the form of a cover on the substrate and has cutouts which accommodate the component structures in regions of the component structures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 08/743,540, nowU.S. Pat. No. 5,831,369, filed on Nov. 4, 1996, which was a continuationof International Application Serial No. PCT/EP95/01658, filed May 2,1995.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to an encapsulation for electroniccomponents, in particular for components which operate by using surfaceacoustic waves (SAW components), having a cap which seals componentstructures on a substrate. The invention also relates to a method forproducing the encapsulation.

Encapsulations and housings for electronic components, in particular forSAW components for radio-frequency applications as well, are preferablyconstructed to be hermetically sealed and include, for example, ametallic cap and a metallic base plate or else a partially metallizedcarrier for the component system. In the case of SAW components, theterm component system in general means a piezo electric substrate havingmetallic structures mounted on it which form, for example, transducers,resonators or reflectors, as well as electrical connections for thosemetallic structures. As a rule, the metallic structures are aluminumstructures which cannot be passivated. After installation in metal ormetal/ceramic housings or encapsulations, short circuits may occur onthe metallic structures. Such short circuits are caused by electricallyconductive metallic particles. Those particles can become detached fromthe inside of the cap or from the metallized regions of the componentsystem carrier. Furthermore, the soldering or welding of the cap andbase plate or component system carrier may be a source of suchConductive metallic particles, since solder or welding spatter, whichcannot be completely avoided, may lead to short circuits on the metalliccomponent structures. Finally, it is also possible for metallicparticles to be produced when making contact between wires and theelectrical connections of the metallic component structures.

The problem of short circuits resulting from conductive metallicparticles of the type mentioned above can be avoided, for example,either by avoiding the production of the particles, by removing existingparticles, or by such particles being reliably fixed at safe points.

The particles are predominantly formed on caps and metallic base platesduring the production of the caps, for example by friction duringrolling or in a thermoforming and stamping tool. Attempts may be madethrough the use of intensive cleaning processes to keep the number ofparticles as low as possible or to coat the inside of the caps beforeassembly, for example with a polymer which binds and fixes theparticles. However, polymer coatings are disadvantageous since theirapplication onto defined regions, for example in the case of polygonalcaps, is difficult in production terms, complete coating of the innersurface is not possible, because soldered or welded edges and regions ofthe heat influence zone during soldering or welding must remain free,and space problems caused by relatively thick layers and outgassing fromthe polymer adversely affect the long-term functionality of thecomponents.

Furthermore, metallic coatings that are composed, for example, ofnickel, may be applied electrochemically or non-electrochemically(chemically) in order to fix particles in caps or on metallic baseplates. However, it is not possible to achieve complete freedom fromparticles, even in that way.

In the case of ceramic component system carriers, it is not possible tocompletely avoid particles of the metallization breaking off or becomingdetached, particularly on relatively sharp edges, even when, with thatin mind, the component system carrier is constructed optimally, forexample by rounding the edges or ending the metallization before theedges.

Furthermore, with the present-day standard, it is impossible toimplement wire contacts completely without any friction.

Passivation of the component structures on the substrate through the useof insulating, sufficiently thick layers immediately on the structuresis not possible as a rule in the case of high-precision components,since the component characteristics are undesirably influenced even byvery thin layers. For example, in the case of SAW components in the formof bandpass filters, that can lead, for example, to a shift in themid-frequency or to an increase in the bandwidth. Compensation for thechange in component characteristics through the use of structures whichare trimmed to take account of those aspects is not always possiblesince, with the present-day standard, it is very difficult to apply thinlayers with sufficient layer thickness reproducibility.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an encapsulationfor electronic components and a method for producing the encapsulation,which overcome the hereinaforementioned disadvantages of theheretofore-known devices and methods of this general type and whichreliably shield metallic component structures against conductivemetallic particles in the case of electronic components, in particularin the case of SAW components, in such a way that electricalcharacteristics, and in the case of SAW components acousticcharacteristics as well, are not unacceptably influenced. This is donesince, as mentioned above, conductive metallic particles which causeshort circuits cannot be completely avoided at an economicallyacceptable cost and direct passivation of component structures ispossible only in exceptional cases.

With the foregoing and other objects in view there is provided, inaccordance with the invention, in a component operating with surfaceacoustic waves (SAW component) and having a substrate and componentstructures on the substrate, an encapsulation for the component,comprising a cap sealing the component structures on the substrate, thecap being formed by a cover on the substrate and the cover havingcutouts formed therein in regions of the component structures foraccommodating the component structures.

In accordance with another feature of the invention, the cover includesan upright carrier surrounding the component structures on the substrateand a covering layer applied onto the carrier.

In accordance with a further feature of the invention, the cover is anintegral element containing the cutouts.

In accordance with an added feature of the invention, the carrier is aclosed frame.

In accordance with an additional feature of the invention, the coverincludes supports in addition to the carrier, the supports beingdisposed on the substrate in regions other than the regions of thecomponent structures.

In accordance with yet another feature of the invention, the cover isbonded, welded or laminated onto the substrate.

In accordance with yet a further feature of the invention, the cover isformed of a material to be structured with a photographic technique.

In accordance with yet an added feature of the invention, the carrierand the supports are formed of a material to be structured with UVlight.

In accordance with yet an additional feature of the invention, thecarrier and the supports are formed of a photoresist material.

In accordance with again another feature of the invention, the coveringlayer is formed of a glass material.

In accordance with again a further feature of the invention, thecovering layer is formed of a glass ceramic.

In accordance with again an added feature of the invention, the coveringlayer is formed of a material to be structured by a photographictechnique.

In accordance with again an additional feature of the invention, thecover exposes or uncovers electrical connections on the substrate.

In accordance with still another feature of the invention, the cover hasopenings formed therein for introduction of an acoustic dampingcompound.

In accordance with still a further feature of the invention, there isprovided a plastic sheath disposed over the cover.

In accordance with still an added feature of the invention, the sheathis formed of a plastic film.

With the objects of the invention in view there is also provided amethod for producing an encapsulation for a component operating withsurface acoustic waves (SAW component) and having a substrate andcomponent structures on the substrate, which comprises forming cutoutsin a cover for accommodating the component structures with the cutouts;sealing the component structures on the substrate with the cover to forma cap; producing a plastic film sheath for the covered substrate bydipping, sintering, potting, extrusion coating or press coating usingplastic compounds on a base of reactive resins or melted thermoplastics;and placing the sheath over the cover.

In accordance with a concomitant mode of the invention, there isprovided a method which comprises producing the cover and the sheathfrom materials ensuring the cutout, after production of theencapsulation, on the basis of mechanical characteristics of thematerials, preferably expansion and shrinkage behavior.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an encapsulation for electronic components and a method for producingthe encapsulation, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, plan view of a part of an encapsulationaccording to the invention for an SAW component;

FIG. 2 is a sectional view of the component according to FIG. 1; and

FIG. 3 is a plan view of a component according to FIGS. 1 and 2, havinga covering layer which is specifically structured for making electricalcontact.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a diagrammatic plan viewof an electronic component that is an SAW component having a piezoelectric substrate 1 on which metallic component structures 10 and 11are provided. The component structures may, for example, be aninterdigital converter 10 and reflectors 11. Such structures are knownper se and will thus not be explained in more detail herein.Furthermore, acoustic damping compounds 12, so-called sinks, may beprovided in specific regions on the piezo electric substrate 1, in amanner that is known per se. The configuration described above isreferred to as a component system herein, for the purpose describedinitially.

In order to avoid the damaging influences of electrically conductivemetallic particles as described initially, an encapsulation is providedon the component substrate 1, according to the invention. According tothe heart of the invention the encapsulation is formed by a cap in theform of a cover 13, 14, 15 (see also FIG. 2 in this context) which hascutouts 16 that accommodate the component structures 10 to 12 (see FIG.2) in regions of the component structures 10 to 12. In the exemplaryembodiment according to FIG. 1, the cover includes a carrier 13 which isconstructed in such a way that it is located at an outer edge of thesubstrate 1. However, this is not absolutely essential according to theinvention. For example, the carrier 13 can also be constructed in such away that the sinks 12 are also located entirely or partially outside thecarrier 13.

According to a development of the invention, the cover (13, 14, 15)includes the carrier 13, which surrounds the component structures 10 to12, on the component substrate 1, and by a covering layer 15 which ismounted onto the carrier. The carrier 13 is preferably constructed as aclosed frame, so that a closed, hermetically sealed encapsulation isproduced in an advantageous manner. A distance between the coveringlayer 15 and the structures 10 and 11 should be of such a size that thecorrect operation of the component is not influenced at any time duringuse. However, the carrier 13 need not necessarily have the form of aclosed frame, but may, if required, also contain apertures through whichparts of the substrate surface are accessible.

According to another development of the invention, in addition toupright carrier 13 in the form of a frame, supports 14 may also beprovided, which contribute to the mechanical support of the coveringlayer 15. According to one embodiment of the invention, the coverforming the cap 13, 14, 15 may be formed by a foil which containsdepressions 16 on the side of the component substrate 1 that cross overthe component structures 11, 12, 13. This can be implemented bystamping, thermoforming or partial material removal from the foil whichis provided for the cover. The foil which is processed in this way canthen be applied onto the component substrate 1, for example by bonding,welding or lamination.

According to a further embodiment of the invention, the cover 13, 14, 15may be formed by a multilayer composite. To this end, a first foil maybe initially formed over the entire surface of the component substrateand be provided with the cutouts 16 by mechanical processing, forexample stamping or another type of perforating operation. The uprightcarrier 13, which is in the form of a frame, as well as the supports 14,if required, are then produced in this way. The covering layer 15 canthen be applied in the form of a prestructured foil or film, or in thefor thin glass, for example by bonding, or melting onto the carrier 13and the supports 14.

However, according to another embodiment of the invention, a materialwhich can be structured by using a photographic technique is used forthe carrier 13, the covering layer 15 and, if required, the supports 14.For example, the material may be a photoresist or a. material which canbe structured by using UV light. This material is then exposed in such away that, after a developing step, only the component structures 10, 11,12 and, in addition, surfaces which are also provided for makingelectrical contact with these.component structures (see FIG. 3), arebare or exposed.

The covering layer 15 is thus applied onto the carrier 13 which has beenproduced in this way, and if required onto the supports 14, and islikewise composed of a material of the type mentioned above which can bestructured by using a photographic technique. This material may beapplied in the form of a solid film which, given an adequate thicknessof the first layer that forms the carrier 13 and, if required, thesupports 14, forms the cutouts 16 with this layer.

Regions 20 may be exposed by photographic structuring of the coveringlayer 15 in order to make contact with the component structures 10 and11, as is illustrated diagrammatically in the plan view according toFIG. 3. Contact pads 21 are thus exposed in these regions 20, for makingcontact by bonding.

After completion of the cover forming the cap which has been explainedabove, further processing of the component can take place in a mannerthat is known per se.

Furthermore, for certain applications it is advantageous for a plasticsheath, which is preferably composed of a plastic film, to be disposedover the cover.

A method for producing such an encapsulation includes the sheathing ofthe covered substrate being produced by dipping, sintering, potting,extrusion coating or press coating using plastic compounds on a base ofreactive resins or melted thermoplastics, which are materials that arepreferably used for covering and sheathing that ensure the requiredcutout, after production of the encapsulation, on the basis of theirmechanical characteristics, preferably their expansion and shrinkagebehavior.

A component according to the invention may be produced in the followingmanner.

After manufacture of a wafer, a substrate (chip) is covered with asuitable plastic film which has cutouts in the region of the componentstructures. This is followed by the chips being separated by sawing,breaking off or the like.

This is followed by the chips being mounted and by the production of theelectrical connections in a known manner in such a way that the chipsare mounted on carrier strips or housing parts and contact is made withthem, for example through the use of bonding wires. Making contactthrough the use of bonding wires can also be omitted if contact areasare kept accessible on the chips (for example for later “chip on board”mounting) or if it is intended to use contact-free signal transmission(for example induction).

These method steps are followed by (complete or partial) sheathing ofthe chips using normal methods such as dipping, sintering, potting,extrusion coating or press coating using materials on a base of reactiveresins or melted thermoplastics. Covering, preferably with a plasticfilm, in this case ensures that the surfaces of the chip with thecomponent structures do not come into contact with the sheathingcompound at all, and do not come into contact with the cover, at leastnot permanently.

If the sheathing compound is processed at sufficiently low pressures,then the cover is not deformed or is only slightly deformed. At the sametime, the enclosed gas cushion between the chip and the cover has asupporting effect since the compression counteracts the cover beingpressed downwards.

At higher processing pressures, it is no longer possible to prevent thecover from being pressed down as far as the chip surface. However, theuse of suitable materials for the cover and the sheath as well asappropriate control of the process conditions ensures that a smalldefined distance (preferably in the μm range) is formed between the chipsurface and the cover once the sheathing compound has cured. This ismade possible in particular by suitable variation and combination of themechanical characteristics of the materials being used (differentlongitudinal expansions and shrinkages that are caused, for example, bytemperature differences and/or chemical reactions).

The individual method steps in the production of a SAW component areindicated in the following exemplary embodiment.

Metallization and structuring of the wafer,

Lamination of the wafer using a resist film which can be structured byUV,

Exposure and developing in order to expose the subsequent cavities and,if required, contact surfaces, etc.,

Lamination of the covering layer from the same resist film on the baselayer produced in this way,

Exposure and developing in such a manner that the cavity remains coveredbut, if required, other points (bonding pads, sawing lines, adjustmentmarks) are exposed,

Press coating of the chips, which are bonded onto a lead frame(carrier), through the use of a conventional method.

During the last method step, the covering film is initially pressed downonto the chip surface over virtually the entire area by the pressure ofthe hot pressing compound (approximately 100 bar). The critical factoris now that, according to the invention, the pressing compound shrinksfar less towards the chip surface than the covering film does away fromit, during cooling from the process temperature. The desired cavity isthus formed in conjunction with good adhesion of the pressing compoundon the covering film, with a height of a few μm above the chip surface.

Finally, it should also be mentioned that openings 17 may be provided inthe covering layer 15, through which sinks 12 of the type mentionedabove and illustrated in FIG. 1 may be directly introduced.

We claim:
 1. A method for producing an encapsulation for a surfaceacoustic wave (SAW) component having a substrate and SAW componentstructures on the substrate, which comprises: producing a cap byproviding a plastic frame-shaped carrier directly on the componentsubstrate with a first photoresist layer applied directly on thecomponent substrate, the carrier accommodating and surrounding the SAWcomponent structures such that at least one cutout is placed in a regionof the SAW component structures for accommodating the SAW componentstructures within the at least one cutout and covering the carrier witha second photoresist layer to encapsulate the SAW component structureson the substrate with the cap; and directly encapsulating the capcovering the SAW component structures onto the substrate with a plasticsheath by a sheathing process that is selected from dipping, sintering,potting, extrusion coating and press coating, using plastic componentson a base of reactive resin or melted thermoplastics.
 2. The methodaccording to claim 1, which comprises producing the cap and the sheathfrom materials having thermomechanical expansion and shrinkagecharacteristics that insure the presence of the at least one cutoutbetween the component structures and the cap after encapsulation.